The present invention relates generally to patient ventilation systems and, more particularly, to a passive silencer as may be used with a blower assembly for a continuous positive airway pressure (CPAP) device. The passive silencer is specifically adapted to reduce the transmission of noise to ambient. CPAP noise includes mechanical noise generated by the rotating components of the blower assembly and air movement noise generated by the passage of air into and through the blower assembly.
One of the most common complaints of CPAP devices and one of the contributors to non-compliance with prescribed CPAP therapy is the excessive noise levels generated by conventional CPAP devices. Such noise is generally unpleasant and objectionable when experienced for extended periods of time. In certain environments such as in an intensive care unit (ICU) of a hospital, the relatively high sound levels generated by conventional CPAP devices is particularly noticeable and can increase anxiety in patients and family members and can affect the mood and concentration of healthcare workers.
Noise-induced stress can have a detrimental effect on the recovery of patients undergoing CPAP treatment. Some studies suggest that noise produced by conventional CPAP devices in a hospital setting may qualify as a health hazard in that the high sound levels can prolong recovery time. Although daytime ambient sound levels are relatively high and can mask some of the noise generated by conventional CPAP devices, such noise is particularly noticeable at night when ambient sound levels are reduced. Depending upon the hearing sensitivity, medication level and cultural background of the particular patient, noise generated by some CPAP devices can interrupt the patient's sleep or prevent sleep altogether.
The Environmental Protection Agency (EPA) and the World Health Organization both recommend maximum noise limits for hospitals of 40 to 45 decibels (dB) during the day and 35 dB at night. However, many conventional CPAP devices generate noise levels that are much higher than EPA recommendations. Recognizing this problem, several devices have been developed which attempt to reduce the objectionable sound levels produced by prior art CPAP devices. For example, commonly-owned U.S. Pat. No. 7,012,346 entitled Miniaturized Electric Motor and issued to Hoffman et al. discloses a motor blower unit for use in a CPAP device.
The motor blower unit is specifically adapted to operate with reduced noise output and includes an impeller mounted on a motor shaft of a motor assembly. The impeller is rotatably coupled to the motor assembly by a bearing assembly. The bearing assembly is sized and configured to minimize the level of mechanical noise produced during rotation of the motor assembly. The impeller is also constructed in a manner that minimizes vibration resulting from static and dynamic imbalances in the motor assembly.
Although the motor assembly is effective in minimizing mechanical noise output that is otherwise transmitted to ambient, a large portion of CPAP-generated noise is a result of air movement into and through the blower assembly. Furthermore, because air movement noise is generally broadband in nature as compared to the generally single-frequency or narrowband nature of mechanical noise, air movement noise is generally more difficult to attenuate.
As can be seen, there exists a need in the art for a CPAP device that reduces the amount of noise that is transmitted to the environment. Furthermore, there exists a need in the art for a CPAP device capable of generating reduced noise levels without a noticeable loss in operating efficiency. More particularly, there exists a need in the art for a CPAP device that reduces noise generated by air movement and which is capable of producing pressurized gas for delivery to the patient at normal pressure settings (e.g., 15-20 cm H2O) and at maximum flow settings (e.g., 80 LPM). Finally, there exists a need in the art for a CPAP device with minimal noise output and which is small in size and simple in construction.